The present invention relates generally to the field of lithium-ion batteries. Specifically, the present invention relates to lithium-ion batteries may be charged at an overpotential to increase the speed of charging.
Lithium-ion batteries include a positive current collector (e.g., aluminum such as an aluminum foil) having an active material provided thereon (e.g., LiCoO2) and a negative current collector (e.g., copper such as a copper foil) having an active material (e.g., a carbonaceous material such as graphite) provided thereon. Together the positive current collector and the active material provided thereon are referred to as a positive electrode, while the negative current collector and the active material provided thereon are referred to as a negative electrode.
FIG. 1 shows a schematic representation of a portion of a lithium-ion battery 10 such as that described above. The battery 10 includes a positive electrode 20 that includes a positive current collector 22 and a positive active material 24, a negative electrode 30 that includes a negative current collector 32 and a negative active material 34, an electrolyte material 40, and a separator (e.g., a polymeric microporous separator, not shown) provided intermediate or between the positive electrode 20 and the negative electrode 30. The electrodes 20, 30 may be provided as relatively flat or planar plates or may be wrapped or wound in a spiral or other configuration (e.g., an oval configuration). The electrode may also be provided in a folded configuration.
During charging and discharging of the battery 10, lithium ions move between the positive electrode 20 and the negative electrode 30. For example, when the battery 10 is discharged, lithium ions flow from the negative electrode 30 to the positive electrode 20. In contrast, when the battery 10 is charged, lithium ions flow from the positive electrode 20 to the negative electrode 30.
FIG. 2 is a graph 100 illustrating the theoretical charging and discharging behavior for a conventional lithium-ion battery. Curve 110 represents the electrode potential for a positive electrode that includes an aluminum current collector having a LiCoO2 active material provided thereon, while curve 120 represents the electrode potential for a negative electrode that includes a copper current collector having a carbonaceous active material provided thereon. The difference between curves 110 and 120 is representative of the overall cell voltage.
As shown in FIG. 2, upon initial charging to full capacity, the potential of the positive electrode, as shown by curve 110, increases from approximately 3.0 volts to a point above the corrosion potential of copper used to form the negative electrode (designated by dashed line 122). The potential of the negative electrode decreases from approximately 3.0 volts to a point below the decomposition potential of the LiCoO2 active material provided on the aluminum current collector (designated by dashed line 112). Upon initial charging, the battery experiences an irreversible loss of capacity due to the formation of a passive layer on the negative current collector, which may be referred to as a solid-electrolyte interface (“SEI”). The irreversible loss of capacity is shown as a ledge or shelf 124 in curve 120.
One difficulty with conventional lithium-ion batteries is that when such a battery is discharged to a point near zero volts, it may exhibit a loss of deliverable capacity and corrosion of the negative electrode current collector (copper) and possibly of the battery case, depending on the material used and the polarity of the case. As shown in FIG. 2, after initial charging of the battery, a subsequent discharge of the battery in which the voltage of the battery approaches zero volts (i.e., zero percent capacity) results in a negative electrode potential that follows a path designated by dashed line 126. As shown in FIG. 2, the negative electrode potential levels off or plateaus at the copper corrosion potential of the negative current collector (approximately 3.5 volts for copper and designated by dashed line 122 in FIG. 2).
During charging of a conventional lithium-ion battery, it may be necessary to take certain precautions to avoid plating of lithium on the negative electrode. For example, as shown in FIG. 2, the potential of the negative electrode (shown as curve 120) approaches a level of approximately 0.1 volts with increasing capacity during charging. For this reason, during charging of conventional lithium-ion batteries, a “taper charge” is applied as the battery approaches a full charge condition to avoid a situation in which the potential of the negative electrode falls below approximately 0.1 volts. With a taper charge, the charging voltage is decreased as the battery approaches a full charge condition to minimize the risk that lithium will be plated on the negative electrode. Further, because of the relatively low potential of the negative electrode, it has been conventionally believed that charge voltages significantly greater than the open circuit voltage of the battery may not be used in charging the battery. As a result, the speed of charging of conventional lithium-ion batteries may not proceed as rapidly as may be desired.
The medical device industry produces a wide variety of electronic and mechanical devices for treating patient medical conditions. Depending upon the medical condition, medical devices can be surgically implanted or connected externally to the patient receiving treatment. Clinicians use medical devices alone or in combination with drug therapies and surgery to treat patient medical conditions. For some medical conditions, medical devices provide the best, and sometimes the only, therapy to restore an individual to a more healthful condition and a fuller life.
It may be desirable to provide a source of battery power for such medical devices, including implantable medical devices. In such cases, it may be advantageous to provide a battery that may be recharged relatively quickly such that inconvenience to the patient may be reduced. It would also be advantageous to provide a battery that may be charged at a greater rate than conventional lithium-ion batteries without significant risk of plating lithium on the negative electrode of the battery. It would also be advantageous to provide a medical device (e.g., an implantable medical device) that utilizes such a battery.